1. Field of the Invention
The present invention relates to a method for heat treatment of coiled springs obtained by cold forming, and in particular relates to an improvement in annealing performed after the cold forming.
2. Related Art
In some kinds of producing methods for a coiled suspension spring used in automobiles, a working method by hot forming or cold forming is used. In the working method by hot forming, a linear wire rod is heated to a temperature of 800° C. or more and is formed into a coiled spring. Then, heat treatment such as quenching and tempering is performed. In the working method by the cold forming, a linear wire rod that was heat-treated in a way such as quenching and tempering (a heat-treated material such as an oil-tempered steel wire) is formed into a coiled shape at ordinary temperature, thereby yielding a coiled spring.
In the working method by cold forming a heat-treated material, since the linear wire rod is formed into a coiled shape, when load is removed after forming, the tensile residual stress remains in the inner portion (the portion facing the axial center) of the coiled spring, and compressive residual stress remain in the outer portion (the portion facing the reverse side of the axial center). These residual stresses largely degrade durability of the coiled spring. Therefore, the stresses are removed after forming the coiled spring by low temperature annealing in a gas furnace or an electric furnace, in which an atmosphere is maintained within a temperature range of 350 to 500° C.
For example, Japanese Patent Application, First Publication No. 55-31109 discloses a method in which low temperature annealing in an atmosphere furnace is performed by applying an oil-tempered Si—Cr steel wire as a heat-treated material. According to the above technique, the average value of the residual stress in portions from the surface to 1 millimeter depth of the oil-tempered steel wire is reduced according to the time for the heat treatment in the furnace. If the time for the heat treatment is more than 30 minutes, the average value of the residual stress hardly varies. For example, Japanese Patent Application, First Publication No. 1-23524 discloses a method in which a linear wire rod is quenched and tempered, and is formed by cold forming into a coiled spring, then stress is removed by annealing. In the above technique, the quenching and the tempering of the linear wire rod before the cold forming are performed by electric resistance heating or induction heating, and the annealing after the forming is performed for a long time in an atmosphere furnace.
In a coiled spring, durability with respect to repeated load is improved according to increase of hardness of the wire rod. Recently, there is great demand for lightweight coiled springs. Therefore, a wire rod that is hard and has great tensile strength is used as a heat-treated material for cold forming, whereby the coiled spring can be used under high stress.
However, when a linear heat-treated material is formed into a coiled spring by cold forming, and the tensile strength of the heat-treated material is large, the residual stress is also large compared to a case in which the tensile strength is low, so that, as is explained in the above, durability of the coiled spring may be degraded.
According to the conventional techniques applied in annealing in an atmosphere furnace, the temperature in the furnace must be raised and the coiled spring must be soaked for a long time to remove the increased stress thereof. However, in the above case, a hardness of the coiled spring is reduced by raising the temperature in the furnace and soaking at high temperature for a long time, and therefore, the amount of the residual stress to be removed must be determined in consideration of balance with respect to the hardness of the wire rod in order to improve the durability of the coiled spring. As a result, the performance of the wire rod cannot be sufficiently functioned.
According to the annealing performed in a conventional atmosphere furnace, since annealing in the furnace requires a long time in a case in which a batch-type atmosphere furnace is used, although heat loss is reduced and energy efficiency can be improved by improvement of air-tightness in the furnace, productivity is reduced. On the other hand, if an open-type furnace for passing through materials is used, although productivity can be improved, the furnace must be made larger and cannot hold the seal because the materials must be transported in and out in heating, whereby heat loss is increased and energy efficiency is deteriorated.
Furthermore, in the beginning of the operation of a furnace, the material cannot be transported into the furnace until the temperature in the furnace is stably maintained at a predetermined temperature, because problems of quality of the product come up. Therefore, the furnace must be heated without the materials inside, so that energy efficiency is greatly deteriorated in a case in which the batch amount is small.